Recently, there has been an increased interest in, and use of, lasers and the light they generate. Generally, a laser comprises a lasing medium enclosed within an optical resonator defined by a highly-reflecting mirror at one end and a partially-reflecting, partially-transmitting mirror at the opposite end. Energy from an external source is applied to the lasing medium so its atoms or molecules are excited, or pumped, to a higher energy state. After the medium is excited, some of the atoms or molecules return to their original, lower energy state in a short period of time. If the difference in energy levels between the two states is large enough, when the electrons or molecules return to their lower energy state, they each release a quantum of light energy, known as a photon.
In a laser, some of these photons reflect off the mirror and stimulate the excited atoms or molecules causing them to move to a lower energy state and simultaneously emit photons, or lase, with the same characteristics as the reflected photons. Some of the photons pass through the partially-reflecting mirror and travel together as a beam of coherent light. The spatial characteristics of this beam are determined by the lasing medium and by the optical resonator and it carries with it the sum of the energy of the individual photons. Lasers are used wherever it is desirable to have a controllable source of light energy that emits a high energy beam of light that can be precisely focused and that has a small cross-sectional area. Since lasers have been developed, applications have been found for them in science, manufacturing, communications and medicine.
One popular type of laser is a gas laser. In this type of laser a gas lasing medium is contained in an elongated space. Often the space is a discharge bore defined by a discharge tube. The mirrors forming the optical resonator are at each end of the space. Cathode and anode electrodes are in contact with the gas so it can be excited by applying an electrical potential therebetween. Gas lasers are popular because, depending on the gas lasing medium used, they can produce a large variety of light with excellent spatial coherence at high power levels.
However, gas lasers have two design considerations, the solutions to which have been mutually exclusive. The first consideration concerns the flow of positively ionized gas ions towards the electron-emitting cathode. This flow, called cataphoresis, tends to cause the gas to move towards the cathode and causes a region of high pressure to form near the cathode and a region of low pressure to form near the anode. Cataphoresis is a concern because it is often desirable to operate a laser in a continuous wave mode where it generates a beam of light for an extended period of time. When a laser is operated in this mode its gas should be at a uniform equilibrium pressure throughout its bore.
Cataphoresis thus makes it necessary to provide lasers with a gas ballast system to prevent uneven pressure build-ups from occurring in the discharge bore. Some ballast systems include an external gas supply and a vacuum pump. These peripherals work in concert to keep the gas within the bore at a uniform equilibrium pressure. However, these peripherals have a disadvantage because providing them adds to both the bulk and cost of the laser system.
An alternative gas ballast system includes a gas return path between the ends of the discharge bore. The system also includes a volume of ballast gas that is in communication with the return path. This technique allows the gas to recirculate around the discharge tube so a uniform pressure is maintained within the bore. Nevertheless, there is a major disadvantage associated with providing a laser with a gas return path. Electrical current tends to flow through the gas return path and excite the gas therein. This is a needless dissipation of energy that could otherwise be used to excite the lasing gas in the discharge bore so a more intense beam of coherent light can be emitted.
These problems have been solved by providing gas lasers with circuitous gas return paths. This significantly decreases the mean free path length (free electron scan travel) so as to inhibit the flow of electrical current within the gas. Providing a laser with this type of path significantly increases the overall size of the laser system. Furthermore, some gas return paths consist of a series of parallel bores drilled in the gas discharge tube. Providing a tube with a number of bores significantly increases the cost of its manufacture. Other systems include gas return paths formed from ceramic or glass tubes that are external to the discharge tube. These tubes add considerable bulk to the size of the laser system. Moreover, these systems are fragile since the extra tubes are prone to breakage. Also, providing these tubes significantly increases the cost of the laser system.
The second design consideration concerns the means used to dissipate heat away from the discharge bore. The processes of exciting the lasing gas generates a significant amount of heat within the bore. In order to insure that the laser can operate continually over a long period of time, this heat must be dissipated away from the discharge bore before it starts to effect the operating characteristics of the laser.
To date, it has been difficult to provide a laser with an efficient, compact gas return path, and an efficient heat dissipation mechanism. For example, a laser with a multi-bore tube containing a discharge bore and gas return paths may have an efficient gas ballast system. However, the thick wall tube needed to contain the bores, and the free space defined by the gas return paths, would inhibit the conduction of heat away from the discharge bore. Alternatively, a large diameter water jacket can be placed around a thin-walled discharge tube with external gas return paths. Providing a laser with such a jacket significantly adds to both the overall size of the laser system, its fragility, and the cost of manufacturing it.
These two design considerations are especially important in the construction of waveguide lasers. A waveguide laser is a gas laser wherein the diameter of the discharge bore is related to the wavelength of light to be emitted. This enables the stimulated photons to reflect off the bore wall and thus maintain a well confined laser beam. As a result, the internal power losses of the lasing gas is minimized and power of the emitted laser beam is maximized. The diameter of the discharge bore in a waveguide laser is generally very small in comparison to that of a conventional gas laser. Cataphoresis and heat build-up affect waveguide lasers more severely then conventional gas lasers because they occur in a much smaller space. Thus, the design of gas ballast and heat dissipation systems takes on new importance with the increasing attention and use given waveguide lasers.
Another consideration in the manufacture of gas lasers is the construction of their discharge tubes. Currently, bulk ceramics, with drilled bores that are extremely smooth straight and regular, are used as discharge tubes. Tubes with bores precisely formed are used because it is felt by those who are experts in the field that emitted photons will not properly guide, or pass through, bores having other characteristics. The disadvantage of using these tubes is that they are relatively expensive to fabricate, especially if long tubes are required, and thus are a major factor in the total cost of assembling lasers.
A need therefore exists for a gas laser that has an efficient gas ballast system and an efficient means for dissipating the heat generated by the gas laser. A need also exists for a gas laser with a more economical gas discharge tube.